This R21 program will fundamentally improve the precision and reduce the cost of magnetic susceptometry for noninvasive measurements of body iron stores. Where existing susceptometers use expensive SQUID sensors that require liquid helium, the new susceptometer will use inexpensive magnetic sensors working at room temperature. In preliminary studies, a first-generation susceptometer with room-temperature sensors showed a good correlation (r=0.98) with an existing SQUID system. The next-generation susceptometer will resolve lower iron levels, in fatter patients, than any existing susceptometer. As a model system, this program focuses on liver iron measurements, which are the best indicator of total body iron. A new susceptometer coil geometry will minimize errors due to the susceptibility responses of the lung and abdominal wall. Errors will be reduced further by evaluating and correcting for the lung and abdominal-wall susceptibilities, as well as improving the water bag that compensates for the diamagnetic background response of the body. These improved susceptometry techniques may ultimately be used to measure iron in the brain, heart and other organs, and to detect tissue-specific magnetic tracers that highlight certain molecules and cell types, signal the presence of cancers, or indicate the function of the lymph nodes, liver and spleen.